The present invention relates to an electron beam instrument for generating an electron beam in the form of a hollow cone which is directed onto a specimen to observe it.
Dark-field illumination has long been used in which a filmy specimen is irradiated with an electron beam that is emitted from an electron beam instrument at some angle to the optical axis. Of the scattered electron wave (beam) which has passed through the specimen, the wave traveling in the direction of the optical axis is collected to obtain a dark-field image of the specimen. In this method, only a part of the scattered wave contributes to the formation of the image and, therefore, the image is not sufficiently bright. In addition, it has the disadvantage of directivity.
In recent years, some attempts have been made to collect all the scattered wave having the same scattering angle to produce an image. In particular, referring to FIG. 1, a shade plate M formed with an annular aperture A is used, and an electron beam EB passed through the plate M is directed onto one point P on a specimen. In another attempt, as shown in FIG. 2, an electron beam is deflected by two stages of deflection systems 1a and 1b, and the azimuth angle .phi. is swept while maintaining both the incident point on a specimen 2 and the tilt angle .phi. substantially fixed, thus creating substantially the same situation as in the case where an electron beam in the form of a hollow cone falls on one point on the specimen. In either case, the scattered wave passed through the specimen is collected to form an image of the specimen. This technique is described, for example, in "The Proceedings of the Electron Microscopy Society of America", held in Phoenix, Ariz., Aug. 8-12, 1983, pp. 406-407. Although the type of instrument shown in FIG. 2 has the advantage that the tilt angle .theta. of the electron beam entering the specimen can readily be changed, it cannot sufficiently correct the astigmatism of the object lens and hence cannot yield images with high resolution. This problem arises because the direction of the astigmatism varies with the direction of the tilt, or the azimuth angle, of the electron beam EB.